In recent years, there have been held electronic conferences in which an electronic device such an electronic whiteboard equipped with a touch panel is used so that a plurality of users can input information about the conference and progress the conference with looking at the information. In such electronic conferences, users progress a discussion while freely inputting information (performing operations) on the electronic device, which sometimes causes a situation that the display of the electronic device has to be restored to the former state and a situation that the former state at a certain point in time has to be reproduced. To address this issue, such the electronic device is configured to record a series of operations and reproduce a state of the display at a certain point in time by undoing the operations.
As a technique about the above-mentioned undoing of operations, for example, Japanese Laid-Open Patent Application Publication No. H07-200226 discloses a visualizing device of an operation sequence. The device includes an operation recognizing section configured to convert an operation performed by a user into a command, and an operation-sequence recording section configured to store the commands converted by the operation-recognition section as an operation sequence. The device further includes an operation-sequence converting section configured to convert the operation sequence into a directed graph, an operation-sequence displaying section configured to display the directed graph in an editable form, and an operation-sequence executing section configured to re-execute the operation sequence, based on the directed graph. This publication discloses that the device converts a history of the operations, which was performed by a user on the data, into a directed graph which is easy to be edited and reused by the user, and presents the directed graph to the user.
Concretely, in a conference system using an information display apparatus such as an electronic whiteboard, for example, when users have a discussion on a first point of argument with performing operations of serially writing pieces of information (for example, objects such as characters and figures) on a screen of the device, it sometimes is desired to discuss again a second point of argument while tracing back to a certain point of time. Further, it is sometimes desired to trace back to a certain point of time to correct the written information and to discuss again the first point of argument.
In these cases, it is necessary to trace back to an intended point of time. However, a conventional information display apparatus is configured to only store a series of operations in order, and if another operation has been performed after going back to a certain point of time, the conventional information display apparatus hardly reproduces the state of the screen at the intended point of time just by using a method of serially undoing operations. Further, in Japanese Laid-Open Patent Application Publication (JP-A) No. H07-200226, it is described that the conventional information display apparatus is configured to display an operation performed after an undo operation as a branch of the original operation before being undone. However, for example, the conventional information display apparatus is configured to also deal a history of a branch which was purposely performed for a certain purpose, as a branch which was purposelessly undone with this arrangement; thus, with such a display method, it is difficult for users to appropriately determine what time point to trace back, whereby there is a problem that it is difficult for users to reproduce a desired state. The present invention seeks to solve the problem.